Powder coating is a well known method for coating of objects in which one starts with a powder coating material which is electrically charged and sprayed against the surfaces of an object, and which material is finally adhered and converted to a solid state by heating to its melting temperature. Since the powder consists of a plastic which is cured by heating, it must be heated to a comparatively high temperature, about 200.degree. C.
This coating method may be performed on objects having good heat resistance and a conductive surface. However, where the surface of an object is non-conductive, implying that the object cannot be grounded or supplied with a charge of an opposite polarity to the charge of the powder, difficulties arise with getting the powder to adhere to the surface of the object during the time between spraying and heating the powder to the melting temperature.
The difficulty of obtaining a polarity difference between the powder and the object, when non-conductive surfaces are involved, has in certain processes been addressed by either varnishing the object with a conductive varnish, or subjecting it to water so as to form a conductive moisture layer on the surface. These methods are, however, of limited use because of disadvantages including the additional operation and material required by varnishing, and inferior adhesion as compared to powder coating on a clean surface. Additionally, when utilizing such methods, discoloration may occur with clear varnishes.
The addition of water may impair the adhesion of the powder coating and can damage the object by confining the added water beneath the coating.
A further method of getting the powder to adhere to the surface of a non-conductive object is disclosed in Albers, German Patent No. 3,211,282. Albers teaches heating glass objects having good heat resistance to a temperature of 400-900.degree. C. This causes the powder granules which impact the object to melt and stick to the surface, making it possible to bring the conversion to a homogeneous, solid state to an end. Objects that are heat sensitive may deform when exposed to high temperatures and cannot be treated at the high temperature required by this method. Thus, the method of Albers cannot be applied to heat sensitive objects such as wood or plastic.
An object of the present invention is thus to provide an apparatus and method for polymeric powder coating of heat sensitive objects.